Virtually all computers use some type of screen as their primary output device. There are two categories of screen: cathode ray tube and LCD.

CRT screens: The cathode ray tube (CRT) type screen is usually called a monitor and makes use of the same technology as a television screen. A beam of electronics is fired from an electronic gun at the back of the tube. This strikes the front of the tube which is covered in a phosphorescent material which glows when struck by electrons. Between the electron gun and the screen the beam is modulated by a signal to produce the image you see on the screen.

With CRT type screens, an important measure is the refresh rate. Roughly speaking, this is the number of times the image is refreshed every second. A low refresh rate makes the image appear to flicker. You need a refresh rate of at least 72 Hz (72 times a second) to avoid the appearance of flicker.

Solid state screens: Solid state screens, also known as LCD or Liquid Crystal Displays, make use of tiny transistors to emit light and create an image. Originally, LCD screens were confined to laptops, but they are increasingly used with desktops. They are usually called flat screens when used as separate units with desktops.

Resolution: An important characteristic of all screens is their resolution. Each point of light on the screen is called a pixel. The resolution of a screen is the maximum number of pixels that the screen can display. This is given as the number of pixels across (horizontal resolution) by the number of pixels down (vertical resolution). For example, 800 x 600. The greater the resolution the better. Modern screens can display 1024 X 768 or better.

Printers produce a hard copy of the output on paper. There are three main types of printer: Dot matrix, Inkjet and Laser. The following table compares the three types.

A plotter consists of a device that can move paper both backwards and forwards. On the top of the device one or more pens are able to move horizontally across the paper. The combined movement of the pens horizontally across the paper and the vertical movement of the paper allows complex continuous diagrams to be drawn.

Some plotters allow different colour pens to be used to create diagrams in multiple colours.

In other types of plotters, the paper lies on a flat bed. The mechanics of the plotter are so designed that the pens can move both across and down the paper to create the diagram.

Plotters are usually used in conjunction with CAD (Computer Assisted Design) programs. These are used in everything from the design of ships and machines to buildings.

Modern computers using the appropriate software can turn text in a document into audible speech. This is known as speech synthesis. Other types of software allow music and other sounds to be created and played back.

The line between the computer and a home entertainment system is becoming blurred. Computers are able to play music directly from a CD or play a film from a DVD. You can even fit your computer with a radio or TV card to add these functions.

In all cases, the sound is transmitted through a speaker in the same way it is in a sound system or radio.